The present invention relates to an active-matrix-type liquid crystal display device, such as a thin-film-transistor (TFT) liquid crystal display device, a data signal line driving circuit, and a liquid crystal display device driving method.
Recently, liquid crystal display (LCD) devices have rapidly become in common use since consumed power is small and the size can be easily reduced, as compared with CRT (cathode-ray-tube) display devices. Among such LCD devices, active-matrix-type LCD devices that are characterized by quicker response and easier multiple-gray-level display are widely used.
In a conventional active-matrix-type LCD device 101 as above, for example, as shown in FIG. 13, when a scanning signal line driving circuit 104 selects a scanning signal line GLj, field-effect transistors SW shown in FIG. 2 provide conduction at pixels PIX connected with the scanning signal line GLj, thereby connecting the pixels PIX(i,j) with data signal lines SLi corresponding to the pixels. On the other hand, a data signal line driving circuit 103 outputs display data D to data signal lines SL1 through SLn based on video signals DAT so that the display data D are to be fed to the pixels PIX. Charges corresponding to respective differences between outputs of the data signal lines SL1 through SLn and a common electrode potential Vcom are stored in pixel capacitors CP of the pixels PIX. At pixels PIX connected with the scanning signal lines GL that are not selected, switching elements SW thereof are opened, thereby holding charges in pixel capacitors CP thereof. Incidentally, transmittance of liquid crystal elements varies with a voltage applied. Therefore, while consecutively selecting the scanning signal lines GL1 through GLm, the display data D are written in a pixel PIX(i,j) during a selection period of each scanning signal line GLj. By so doing, the LCD device 101 causes an image according to the foregoing video signal DAT to be displayed on the liquid crystal panel 102.
In the foregoing active-matrix-type LCD device 101, the data signal line SLi and the pixel capacitor CP are separated while the scanning signal line GLi is not selected, and a voltage according to the display data D that have been written in the pixel capacitor CP upon selection is continuously applied to the liquid crystal element. Therefore, as compared with a simple-matrix-type LCD device, the multiple-gray-level display can be relatively easily realized.
The foregoing arrangement, however, undergoes a problem such that, to realize a higher-definition active-matrix-type LCD device with a larger display screen particularly, horizontal shadow more easily occurs, impairing image quality.
More specifically, taking the case where polarities of outputs of the data signal lines SL1 through SLn are reversed every horizontal scanning period, current flows to charge/discharge a capacitor between a source of each field-effect transistor SW and a common electrode Tcom every horizontal scanning period. Note that examples of such capacitors include, apart from the foregoing pixel capacitors CP, capacitors formed between the data signal lines SL1 through SLn and the common electrodes Tcom, cross capacitors formed between the data signal lines SL1 through SLn and bus lines, and cross capacitors formed between the data signal lines SL1 through SLn and the scanning signal lines GL1 through GLm.
Here, current charging/discharging the foregoing capacitors varies with output amplitudes of the data signal lines SL1 through SLn. Therefore, in the case where resistances between supplementary capacitors CS of the pixel capacitors CP, common transfer resistances, output impedance of a common electrode driving circuit 105, etc. cause resistance components to exist in the common electrode line COM connected with the common electrodes Tcom and CS bus lines connected with the supplementary capacitors CS, a voltage fall due to the foregoing resistance components varies with the output amplitudes of the data signal lines SL1 through SLn. Consequently, a rising speed of a common electrode potential Vcom waveform varies with a display pattern that varies every horizontal scanning period.
For example, as shown in FIG. 14, comparing a portion A in which all the data signal lines SL1 through SLn output a white level during one horizontal scanning period with a portion B that includes an output of a black level with a greater potential difference than that of the white level with respect to the common electrode potential Vcom, current flowing at a root part of the common electrode line COM and root parts of the CS bus lines is greater in the portion B than in the portion A. Therefore, the rising of the common electrode potential Vcom waveform is duller in the portion B as indicated by a broken line than in the portion A as indicated by a solid line in FIG. 15.
Here, in the case where the charging period for the pixel capacitors CP is sufficient, charging voltage levels to the pixel capacitors CP are equal to each other in the portions A and B. In the case where, however, for example, the charging to the pixel capacitors CP is not completed during the charging period due to insufficient driving capacity or operating speed of the field-effect transistors SW, charges less than the value indicated by the display data D are provided to each pixel capacitor CP, and are maintained during a non-selection period as well. In this case, charging becomes insufficient in the portion B rather than in the portion A. Consequently, the brightness of a white part of the portion B becomes higher than the brightness of a white part of the portion A, resulting in that white horizontal shadow occurs. Incidentally, the explanation herein is made by using a normally-white-type LCD device, but the same applies in the case of a normally-black-type LCD device.
The foregoing horizontal shadow can be prevented by reducing the resistance components of the CS bus lines and the common electrode line COM and by ensuring sufficient charging period for charging the pixel capacitors CP. However, there are limits to reduction of the resistance components and improvement of characteristics of the field-effect transistors SW, while higher-definition LCD devices with larger display screens are demanded. Enlargement of the display screen will require longer CS bus lines and common electrode line COM, thereby making it difficult to reduce the resistance components. Besides, in a high-definition LCD device, data signal lines SL1 through SLn and the scanning signal lines GL1 through GLm increase in number, making it difficult to ensure sufficient charging time. Therefore, in such LCD devices in particular, horizontal shadow more often occur, and elimination of horizontal shadow is demanded.
Incidentally, the U.S. Pat. No. 2,960,268 (Date of Publication: Jul. 8, 1994) discloses an active-matrix liquid crystal panel including capacity-coupled sensing electrodes that cross data signal lines SL1 through SLn with an insulating film provided between the same and the data signal lines SL1 through SLn, and an inverter for applying to the common electrode a voltage that corresponds to a potential fluctuation occurring to the sensing electrode and that is obtained by reversing a polarity of the potential fluctuation. This arrangement is aimed to cancelling the potential fluctuation occurring to the common electrode with the voltage applied to the data signal lines SL1 through SLn, so as to prevent occurrence of horizontal shadow. In the present arrangement however, application of the output signal of the inverter to the common electrode for driving the common electrode makes AC driving impossible, and causes the power consumed by the whole LCD device to drastically increase. On the other hand, as described above, it is preferable that power used for removing horizontal shadow is small, since the LCD devices are often used in fields where reduction of power consumption is demanded as described above.
An object of the present invention is to provide an active-matrix-type liquid crystal display device capable of preventing horizontal shadow with lower power consumption.
To achieve the foregoing object, an active-matrix-type LCD device of the present invention includes (i) a coupling section for generating a coupling signal in accordance with a sum of the outputs, based on outputs to the data signal lines, and (ii) a common electrode driving circuit for, based on the coupling signal and the driving signal as a reference used in generation of the common electrode signal, generating the common electrode signal that is given an influence such as to suppress potential turbulence caused by an output to each data signal line, by comparing the common electrode signal with a common electrode generated according to only the driving signal.
According to the foregoing arrangement, based on outputs to the data signal lines, a coupling signal is generated in accordance with a sum of the outputs, and a common electrode signal is generated based on the coupling signal and the driving signal. This enables to apply to the common electrode of each pixel an influence that corresponds to a fluctuation of a potential of the common electrode due to the outputs of the data signal lines and that is in a direction opposite to that of the fluctuation, with electric power lower than that in the case where the foregoing coupling signal is directly applied to the common electrodes. Besides, the same voltage waveform can be applied to the common electrode of each pixel, irrespective of display patterns. Furthermore, since the common electrode driving circuit generates the common electrode signal based on the coupling signal and the driving signal, the driving capacity and output range of the coupling section can be suppressed, as compared with the case where the foregoing coupling signal is directly applied to the common electrodes. Therefore, the power consumption can be lowered, and even in the case where a sufficient time for charging the pixel capacitors cannot be secured, horizontal shadow can be prevented.
Furthermore, to achieve the aforementioned object, an active-matrix-type LCD device in accordance with another preferable embodiment of the present invention includes (i) a coupling section for generating a coupling signal in accordance with a sum of the outputs of the data signal lines by the switching cycles of the outputs, according to the display data, and (ii) a common electrode driving circuit for, based on the coupling signal and the driving signal as a reference used in generation of the common electrode signal, generating the common electrode signal that is given an influence such as to suppress potential turbulence caused by an output to each data signal line, by comparing the common electrode signal with a common electrode generated according to only the driving signal.
According to the foregoing arrangement, based on display data for generation of outputs to the data signal lines, the coupling section generates a coupling signal in accordance with a sum of the outputs, while the common electrode driving circuit generates a common electrode signal based on the coupling signal and the driving signal. Therefore, as in the case where the common electrode signal is generated based on the outputs of the data signal lines, this provides reduction of power consumption and prevention of horizontal shadow even in the case where a sufficient time for charging the pixel capacitors cannot be secured.
Furthermore, since a sum of the outputs is obtained not only according to the outputs of the data signal lines but according to the display data, horizontal shadow can be prevented without changing the arrangement of the data signal line driving circuit and the liquid crystal panel.
Furthermore, in each of the foregoing arrangements, the coupling section may preferably include a coupling circuit for coupling the driving signal with the coupling signal, and the common electrode driving circuit amplifies the driving signal coupled with the coupling signal, so as to generate the common electrode signal.
In the foregoing arrangement, the driving signal is amplified by the common electrode driving circuit after the coupling operation of the coupling signal by the coupling circuit, thereby becoming the common electrode signal. As a result, in spite of a relatively simple arrangement in which a coupling circuit is added to the arrangement for generating a common electrode signal by amplifying the driving signal, the common electrode signal, which is generated using the driving signal as a reference, can be controlled according to the coupling signal.
Furthermore, in the foregoing arrangement, the coupling circuit is preferably a coupling capacitor. With this arrangement, since the coupling operation of the coupling signal is carried out by the coupling capacitor, which is a passive element, the power consumption of the LCD device can be suppressed as compared with the case where the coupling operation is carried out by an active element.
Furthermore, in the foregoing arrangement, the driving signal is preferably applied to the common electrode driving means via a resistor, and a time constant according to the coupling capacitor and the resistor is set so that an extent of coupling between the coupling signal and the driving signal should have a predetermined value. With this arrangement in which the extent of coupling is set according to the time constant according to the coupling capacitor and the resistor, the common electrode signal can be controlled in accordance with the coupling signal, in spite of the simplicity of the arrangement that does not utilize a high-performance operational amplifying element.
Additionally, in the foregoing arrangement, the LCD device preferably further includes adjusting circuit for adjusting at least either a resistance of the resistor or a capacitance of the coupling capacitor. This arrangement allows each of LCD devices to, by means of its own adjusting means, adjust the extent of coupling according to a magnitude of shadow occurring thereto, so that the shadow can be prevented. Consequently, even in the case where the LCD devices too greatly vary, it is possible to make the LCD devices capable of preventing occurrence of horizontal shadow.
On the other hand, a method for driving an active-matrix-type liquid crystal display device in accordance with the present invention is characterized in that the common electrode signal is dulled as a potential difference between the common electrode signal and a sum of outputs of the data signal lines becomes smaller by the switching cycles of the output signals.
Therefore, an influence that corresponds to a fluctuation of a potential of the common electrodes caused by outputs of the data signal lines and that is in a direction opposite to that of the fluctuation can be given by the common electrode signal. As a result, irrespective of display pattern, a voltage waveform dulled in the same manner is applied to the common electrode of each pixel. Consequently, even in the case where a sufficient period for charging the pixel capacitors cannot be secured, the occurrence of horizontal shadow can be prevented with low power consumption.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.